New mod... sort of
I thought of something that could prove very useful. I was thinking it might be helpful to have a Java app that allows you to layout your map without having to load up Hammer or the PeTI. It would be a top-down view of the "field". The interface would allow you to place items around the map just like PeTI does, but it would only be so you could get a basic idea of what things work and what things don't. It would essentially be like doodling on paper. For an idea of how it works, Google "Javascript Redstone Simulator". That is the basic idea behind what I am talking about. I'm sure someone on here could design such a thing for Portal 2.
yes.. someone could... but at that point why not just go use the PTI and look down?
The point is to make the process quick and simple. The Java applet would be designed to act as digital paper. I like to draw the level on paper before I ever even start in PeTI, much less Hammer. I am sure there are others who do the same. This applet is meant to be the digital equivalent of pen and paper. it is not meant to replace PeTI. Also, PeTI requires an investment of time and energy that this applet does not.
PeTI requires that you start Portal 2 then open the editor. Even when you are in the editor, you have to have some idea of what you want to put down. The Java app is meant as a place to make some quick notes when you have an idea. It would take a few seconds to start and use the thing, unlike PeTI which can require several minutes at a minimum.
Why not just use pen and paper then? 
I like to design a map top-down on paper (I'll scan in a recent design in a minute), as you said, then just load up Hammer and place all the floor brushes I need. Then I switch to a side view and add walls and proper heights and such.
Or if I don't have a good idea in advance, I just place brushes and entities until the puzzle works. But the first way tends to produce better maps.
PTI takes time?
pshh, personally I have a hard time making puzzles in the PTI... my creative process is as follows
pick an element, open hammer, make a puzzle... ideas just flood my head while I'm in hammer.. I get no ideas in the PTI and rarely any with pen and paper
I thought of this thing because I currently have no access to my copy of Portal 2. But the point of the app was to let people who like pen-and-paper design be able to continue designing that way while also being able to see how there idea might work programmatically. Yes, it can be done in PeTI or Hammer. Sure, you can just start throwing things at the wall and see what sticks. This program is meant to be another tool to help bring you vision to life.
You're welcome to make it. No issues here.
how would you show different items?... would we need sprites for everything.. how would things go on the walls?
how would you show portable walls if this is all top down
IMO if you give it a 3d feel to it, then it would just be the PTI
and if you just do side views, then it becomes a worse version of hammer
before anyone could make this application, they'd need design specs
I usually show which item I am using with a letter/number combination, i.e. B3 indicates the third button placed. I suspect it would relatively easy to use some sort of simple sprites like the redstone simulator I mentioned previously does.
Things that are mounted on the walls in my drawings are indicated with a half circle, but in java they could simply occupy the grid space inside the walls since the walls would be one unit wide on the grid.
There could be layers to allow for items to be placed at varying heights. Since PeTI (and to a lesser extent, Hammer) uses a voxel approach to placing items, the java program could assume that the items will be placed within a 64 unit grid.
The background of the grid would be one color. The grid would be another color. The walls would be black when not portalable and white when portalable. The grid and its background would be some color that contrasts with everything else to allow for easy visabilty. Of course, all the colors could be customizable.
and if you just do side views, then it becomes a worse version of hammer
It need not be 3D. Side views are not absolutely necessary (or even desired).
would you like to draw an example in paint?